1. Field of the Invention
The present invention relates to a leak power ratio detection circuit for detecting an adjacent channel leak power ratio of a radio signal transmitted from, for example, a CDMA (Code Division Multiple Access) type mobile communication terminal.
2. Description of the Related Art
Amplitude modulation components are processed in uplink modulation methods of modern digital mobile communication systems, such as π/4 shift QPSK applied to PDC (Personal Digital Cellular) systems, OQPSK applied to cdmaOne systems, and HPSK applied to W-CDMA systems and cdma2000 systems.
A transmission power amplifier in the mobile communication terminal of this type is required to linearly amplify a transmission signal waveform including an amplitude variation component. However, the power consumption of the transmission power amplifier is disadvantageously increased in order to keep the linearity. In particular, in a case where the terminal transmission power is lowered because the mobile communication terminal is present near a base station, most of the power supplied from a cell to the power amplifier will be wasted.
In order to solve this problem, an idea is proposed that when a transmission power is not large, the supply power to the power amplifier is decreased in accordance with the level of the transmission power.
In general terms, if the power supplied to a power amplifier is excessively decreased, the linearity needed for an output signal level is lost, which leads to deterioration in adjacent channel leak power.
If consideration is given to the change in linearity due to a variance in manufacture of power amplifiers and an ambient temperature variation, it is difficult to control power supply to the power amplifiers. In order to avoid this advantage, it is a general method to decrease the degree in reduction of the supply power to the power amplifier. This method, however, fails to sufficiently decrease the power consumption.
Another possible method is to extract an adjacent channel leak power component from a radio transmission output signal using a filter, measure the level of the extracted leak power component, calculate a ratio between this level and the transmission output level to find an adjacent channel leak power ratio, and properly reduce the supply power to the power amplifier using information of this ratio.
In the mobile communication system, however, the frequency channel to be used is changed depending on cases. Thus, the center frequency of the aforementioned filter needs to be so varied as to be able to extract adjacent channel leak power components in association with all possible radio frequency channels. It is difficult to realize such a filter in the mobile communication terminal.